This invention relates to improved latch mechanisms for releasably holding a door or the like in closed condition. The invention is in certain respects especially effective for use in conjunction with swinging doors, and to simplify this disclosure will be described and shown as applied to that use but without limiting the scope of the claims to preclude coverage of the mechanism when employed for latching other types of moveable members for which the mechanism may be adapted.
The conventional type of door lock employed in the construction industry includes a spring urged latch bolt which is moveably carried by the door, and is adapted to engage and be deflected by, and co-act in holding relation with, a keeper carried by an adjacent jamb. Knobs carried by the door release the latch bolt to permit opening of the door, and may be controlled by a key operated lock unit preventing such release of the bolt.
One difficulty which has been encountered with such prior conventional door locks has been the relative ease with which an unauthorized person can, from the outside of the door, move the latch bolt to the retracted position without use of a key. More specifically it is relatively easy with most of these prior locks to insert a thin element such as a knife blade, credit card or any of numerous other similar items which may be readily available, into the narrow space between the edge of the door and the adjacent jamb in a manner contacting the inclined cam or wedge face at the outer side of the latch bolt and exerting force there against to deflect the bolt to its retracted position permitting opening of the door. Because the cam face of the latch bolt is at the side of the bolt facing toward the outside of the door any appropriate instrument inserted between the door and jamb can easily contact that surface and deflect it to a retracted position with little or no skill required.
Various expedients have been devised in the past for attempting to prevent such unwanted retraction of a latch bolt of the above discussed type. For example, in some locks currently on the market a second spring pressed element is provided adjacent the latch bolt proper, and is deflected by the keeper, when the door is closed, to a position in which it is intended to block such retraction of the bolt by other than the key actuated unit. However, the prior devices of this type of which I am aware are not as affective as would be desired, and can usually be defeated with only very little more manipulation than when the second spring pressed element is not present. An unauthorized person can still exert retracting force against the cam face of the latch bolt by a thin card or element as discussed, and in most cases release it for retraction by jiggling the door while such force is applied.